Ultrasound imaging systems generate and transmit ultrasound signals. Typically, the transmitted ultrasound signal is a pulse with a center frequency within the 2 to 15 MHz range. The ultrasound signal propagates through a body, such as the chest cavity of a patient. The transmitted ultrasound signal reflects off structures within the body, such as blood cells or tissue boundaries. Some of the reflected signals, echo signals, propagate towards the transducer.
As the transmit signal propagates through and scatters within the body, additional frequency components are generated, such as at harmonics of the transmit frequency. These additional frequency components continue to propagate through and reflect off structures in the body. Echo signals having the same frequencies as the transmit signal and echo signals associated with the additional frequency components impinge on the transducer. The additional frequency components are caused by non-linear propagation and non-linear scattering or reflecting.
The echo signals are detected and processed by the ultrasound system. Conventional ultrasound systems filter or otherwise remove signals having harmonic or different frequencies than the transmit frequencies. Thus, echo signals resulting from linear propagation and reflection are detected by the ultrasound system.
The detected linear echo signals are used to create an image. For example, Doppler and B-mode image information are derived from the linear echo signals. The information is then used to generate two-dimensional or graphical displays.
As an alternative and as a discussed by Ted Christopher in "Finite Amplitude Distortion-Based Inhomogeneous Pulse Echo Ultrasonic Imaging", IEEE Trans. UFFC 44(1), pp. 125-139 (January 1997), harmonic echo signals, signals resulting from non-linear propagation and scattering, are detected. The echo signals having the same or similar frequency band as the transmit signals are filtered or otherwise removed. An image, such as a B-mode image, is generated from the detected harmonic echo signals. Typically, the signal-to-noise ratio of the echo signals at harmonic frequencies may be acceptable for B-mode imaging, yet unacceptable for Doppler imaging. Various techniques for non-linear imaging are disclosed in U.S. Pat. Nos. 5,410,516 and 5,255,683.